1. Field of the Invention
The present invention relates to an improvement in an internal-combustion engine which has combustion chamber consisting of a main combustion chamber and a sub-combustion chamber to burn lean air-fuel mixture.
2. Description of the Prior Art
Heretofore, there has been publicly known a method such that air-fuel mixture with high air-fuel ratio is supplied to an internal-combustion engine (hereinafter sometimes referred to merely as "engine") to take place the combustion of lean air-fuel mixture in the engine, in order to decrease effectively and simultaneously the contents of CO (carbon monoxide), HC (unburnt hydrocarbons), and NOx (nitrogen oxides) which are harmful components contained in the exhaust gas of internal-combustion engine. However, such a lean air-fuel mixture has an inferior ignitability and a low speed of combustion, resulting in a degraded engine efficiency and an unfavorable rate of fuel consumption (g/horsepower .multidot. time).
An internal-combustion engine having a main combustion chamber and a sub-combustion chamber is publicly known, which is designed for eliminating the disadvantage described above. In this engine with sub-combustion chamber, the combustion chamber is divided into a main combustion chamber and a sub-combustion chamber in communication with the main combustion chamber through a passage. The air-fuel mixture formed in the sub-combustion chamber is initially ignited by a sparking plug. On account of the pressure rise due to the combustion of the air-fuel mixture in the sub-combustion chamber, the combustion gas in the sub-combustion chamber is discharged at a high velocity and over a large area through said passage into the main combustion chamber so that the air-fuel mixture in the main combustion chamber burns very rapidly, and thus the speed of combustion of the air-fuel mixture in the main combustion chamber is increased and the engine efficiency is prevented from being degraded.
The internal-combustion engine having a sub-combustion chamber is classified into two types, i.e., the stratified combustion type and the homogenious lean mixture combustion type. The stratified combustion type of internal-combustion engine, uses an auxiliary intake valve or the like for introducing a readily ignitable rich air-fuel mixture into the sub-combustion chamber so that the rich air-fuel mixture is initially ignited and burns. Some automobile manufacturers are producing this type of internal-combustion engine on a commercial basis, and a fairly large number of the engine is being marketed. This type of internal-combustion engine having a sub-combustion chamber is capable of performing a very sure igniting operation and can run satisfactorily even when the main combustion chamber is filled with an extremely lean air-fuel mixture. However, since two kinds of air-fuel mixtures with different air-fuel ratios must be supplied to the engine, the fuel supply system and the valve mechanisms become much complicated, resulting in such disadvantages as that difficult problems arise in the manufacture of the engine which naturally leads to increased production costs, that the control and adjustment of fuel supply system are rendered very difficult, and that the inspection, repair, and maintenance of the engine require respectively a large number of steps.
On the other hand, in the homogenious lean mixture combustion type of internal combustion engine, the sub-combustion chamber has no means for the introduction of air-fuel mixture such as an auxiliary intake valve and the like, and during the compression stroke of the engine the air-fuel mixture in the main combustion chamber is forced into the sub-combustion chamber through previously described passage to be ignited and burned in the sub-combustion chamber. This type of engine has a very simple construction, and enjoys a high reliability and the ease of manufacture and maintenance. However, to date the actual use of this type of engine has been considered to be nearly impossible for the reason that the ignition is extremely difficult because the lean air-fuel mixture, which in itself is inferior in the ignitability, is adversely affected to a great degree in terms of the ignitability by the burned residual gas existing in the sub-combustion chamber, which were produced by the previous combustion. The applicant of the present invention proposed an internal-combustion engine in which the spark gap of the sparking plug is located in the previously described passage for the purpose of solving the problem described above. In this type of internal-combustion engine, at the moment of ignition just before the end of the compression stroke, the burned residual gas produced by the previous combustion which had been existing in the passage have been carried away into the sub-combustion chamber by the flow of air-fuel mixture existing in the passage during the compression stroke, so that the adverse effects of residual gas on the ignitability can be avoided and the ignition of a lean air-fuel mixture can be performed in a satisfactory manner merely by using a well known high performance ignition system.
In case of the internal-combustion engine having a sub-combustion chamber described above, the interior of the sub-combustion chamber, particularly the passage and its neighorhood, tends to reach a high temperature because the combustion takes place first in the sub-combustion chamber. The homogeneous lean mixture combustion type of internal-combustion engine has a pronounced tendency to such a phenomenon since the lean air-fuel mixture does not have a cooling effect on the interior of the sub-combustion chamber. For this reason, there arises a necessity to construct the sub-combustion chamber with a highly heat resisting material such as stainless steel and the like. However, from the standpoint of production costs and machining operations the cylinder head undergoes, it is impossible to construct the whole body of the cylinder head of an engine with stainless steel. Therefore, there is a need of constructing the sub-combustion chamber with a material different from the material of which the cylinder head is made.
In order to offer a solution to this problem, the applicants of the present invention proposed an internal-combustion engine having a sub-combustion chamber in which a sub-combustion chamber is formed in a sub-chamber element separate from the cylinder head, and said sub-chamber element is forcedly fitted into a recess provided in the cylinder head so that a combustion chamber consisting of a main combustion chamber and a sub-combustion chamber is formed. In this engine, instead of using threaded fasteners and the like, the force fit which enables to obtain a thermal resistance lower than those obtained by the other methods is employed to fix the sub-chamber element in the cylinder head, with the result that the heat dissipates from the sub-combustion chamber in a satisfactory manner, and the troubles such as pre-ignition and the like due to the overheating of sub-combustion chamber is effectively prevented from occurring.
However, when the sub-chamber element is fixed in the cylinder head by means of the force fit, there arises the following problem. During the operation of the engine, the sub-chamber element is heated to a very high temperature, while the temperature of the cylinder head is kept within a certain limited range because the cylinder head is cooled by the cooling water or the like. As a result, a very large stress is produced in the portion of the cylinder head where the sub-chamber element is forcedly fitted due to the difference in thermal expansion between the sub-chamber element and the cylinder head. This stress results in a creep failure and a decreased holding power of the portion of the cylinder head where the sub-chamber element is forcedly fitted. With the increase in thermal resistance, the sub-chamber element is overheated and melts. In some extreme cases, after the engine has cooled the sub-chamber element drops into the main combustion chamber damaging the whole engine. Needless to say, this constitutes a serious disadvantage.